RT Journal Article SR Electronic T1 Oligomerization processes limit photoactivation and recovery of the Orange Carotenoid Protein JF bioRxiv FD Cold Spring Harbor Laboratory SP 2022.02.04.479168 DO 10.1101/2022.02.04.479168 A1 Elena A. Andreeva A1 Stanislaw Nizinski A1 Adjélé Wilson A1 Matteo Levantino A1 Elke De Zitter A1 Rory Munro A1 Fernando Muzzopappa A1 Aurélien Thureau A1 Ninon Zala A1 Gotard Burdzinski A1 Michel Sliwa A1 Diana Kirilovsky A1 Giorgio Schirò A1 Jacques-Philippe Colletier YR 2022 UL http://biorxiv.org/content/early/2022/05/31/2022.02.04.479168.abstract AB The Orange Carotenoid Protein (OCP) is a photoactive protein involved in cyanobacterial photoprotection, by quenching of the excess of light harvested energy. The photoactivation mechanism remains elusive, in part due to absence of data pertaining to the timescales over which protein structural changes take place. It also remains unclear whether or not oligomerization of the dark-adapted and light-adapted OCP could play a role in the regulation of its energy quenching activity. Here, we probed photo-induced structural changes in OCP by a combination of static and time-resolved X-ray scattering and steady-state and transient optical spectroscopy in the visible range. Our results suggest that oligomerization partakes in regulation of the OCP photocycle, with different oligomers slowing down the overall thermal recovery of the dark-adapted state of OCP. They furthermore reveal that upon non-photoproductive excitation, a numbed-state forms, which remains in a non-photoexcitable structural state for at least ∼0.5 µs after absorption of a first photon.Significance Statement The orange carotenoid protein (OCP) is a photoactivatable protein involved in cyanobacterial photoprotection. Upon photoactivation, OCP becomes able to quench the excess of energy uptaken by the light-harvesting antennae, thereby evading damage to the cells. It remains unclear, however, what is the exact OCP photoactivation mechanism, and whether or not oligomerization partakes in the regulation of the OCP function. Here, we investigated these issues by combining static and time-resolved (TR) X-ray scattering and optical spectroscopy. Our results show that OCP oligomerizes in both the dark-adapted inactive and light-adapted active states, suggesting a functional role for oligomerization. TR scattering data furthermore reveal that the first large-scale conformational changes associated with OCP photoactivation take place on the µs time scale.Competing Interest StatementThe authors have declared no competing interest.